1. Field of the Invention
The present invention relates to ventilated enclosures in which the temperature must be maintained at a substantially constant level or must not rise above (or fall below) a predetermined limiting level.
2. Description of the Prior Art
Such enclosures are generally formed either by rooms into which hot or cold air is pumped by means of fans, or by cabinets within which a flow of hot or cold air, which is produced by fans, possibly associated with heating or refrigerating means, is used to heat or cool articles in the cabinets. More particularly, these cabinets may be of the kind which are used to hold the electrical or electronic circuits of a control or regulating installation or some other kind of installation, which circuits dissipate heat in the course of operation and consequently need to be cooled if they are to be capable of operating properly.
In order to allow effective passage for the flow of air produced by the fans, each enclosure thus takes broadly the form of an air duct, one of whose ends is closed off by a partition or a plate pierced with ventilation apertures, and whose other end is closed off by a rigid wall provided with openings which are designed to provide access for the flow of air caused by fans fixed to this wall. An enclosure of this kind may be formed, for example, by a box for connecting printed-circuit boards of the type which is described and illustrated in U.S. Pat. Nos. 3,699,396 and 3,868,158. Advantageously, such an enclosure is provided with at least two fans, which enables ventilation to be continued even when one of the fans breaks down. In addition, the use of a plurality of fans ensures more satisfactory distribution of the circulated air within the enclosure.
In the majority of cases, for example in cases where the enclosure is formed by a cabinet containing electronic circuits, or again in cases where the enclosure is intended to provide a controlled environment for items which do not easily withstand heat or cold, it is also essential for the temperature within the enclosure to remain below or above a predetermined limiting value. This requirement may be met by placing within the enclosure a thermal detection device which generates a signal as soon as its temperature becomes higher than or lower than the predetermined limiting threshold. However, in cases where the enclosure contains electronic components, a detector of this kind often proves unsatisfactory because it fails to detect abnormal rises in temperature in time to protect the components against catostrophic changes in temperature. In effect, the detector, positioned as it is in the stream of air at the outlet, fails to produce a signal until after the air has heated up abnormally from contact with the components which has ceased to be cooled as a result of the stoppage of the fan. The result is that the components may have already been damaged at the time when the signal produced by the detector appears.
To overcome this drawback, it has been proposed to mount on each fan a device for detecting inadequate ventilation which is designed to produce a signal when the speed of rotation of the associated fan falls below a predetermined limiting value. Such a detection device may be formed for example by a dynamo which is coupled to the drive-shaft of the fan and which produces an electromotive force whose magnitude is proportional to the speed of rotation of the shaft and can be measured by apparatus of a known kind. However, this solution has the disadvantage that it involves a relatively high initial cost and that it fails to reveal the reductions in air flow which may occur, even if the fans are turning at their normal speed, when the air-inlet openings opposite which the fans are mounted become partially or totally obstructed as a result of an accumulation of dust or of accidental blockage by an object such as a sheet of paper, for example. In this case, the fans circulate hot air around inside the enclosure.